Process for deep desulfurization by adsorption of a gas oil-type hydrocarbon fraction

ABSTRACT

The invention describes a process for deep desulfurization of a gas oil-type hydrocarbon fraction by adsorption in the presence of hydrogen on a solid that contains zinc oxide and a promoter in oxide form. Said adsorbent solid is regenerated by a controlled combustion with dilute air.

FIELD OF THE INVENTION

This invention relates to a process for deep desulfurization of a hydrocarbon fraction of distillate type or of gas oil according to the terminology of one skilled in the art, using a high-temperature sulfur collection unit.

A distillate-type fraction is defined as a fraction that is obtained from the distillation of crude or from a conversion unit such as catalytic cracking, and whose distillation interval is between 150° C. and 450° C. This fraction may have any chemical nature, i.e., it may have a distribution between any paraffins, olefins, naphthenes and aromatic compounds.

In the text below, we will call this fraction gas oil, but this designation does not have a restrictive nature. Any hydrocarbon fraction that contains sulfur and that has a distillation interval that is similar to the one of a gas oil fraction can be covered by the process that is the object of this invention.

The process according to the invention therefore makes it possible to produce a desulfurized hydrocarbon fraction with contents that are less than or equal to 10 ppm of sulfur, preferably less than 5 ppm of sulfur, and even more preferably less than 1 ppm of sulfur.

The future specifications on automobile fuels call for a strong reduction of the sulfur content in the fuels, and in particular in the gas oils. This reduction is intended to limit the content of sulfur oxide and nitrogen in the automobile exhaust gases. European legislation defines the specifications of sulfur content in the gas oils that since 2000 are 350 ppm in sulfur and will go to 50 ppm of sulfur in 2005 and to 10 ppm of sulfur in 2009.

The evolution of the specifications of the sulfur content in the fuels therefore requires either the improvement of the existing hydrotreatment catalytic processes with the effect of a non-negligible overconsumption of hydrogen and/or an increase in the operating pressure, or the development of new processes for deep desulfurization of gas oils, or a combination of the two.

This invention can be regarded as a new process for deep desulfurization of gas oil fractions that can be applied as of the time that the sulfur content of the feedstock to be treated remains within certain limits or else can be combined with a traditional hydrodesulfurization unit, in which case it will be placed downstream from said hydrodesulfurization unit.

Among the new methods for desulfurization of gas oils, the processes for purification by adsorption of sulfur-containing compounds on a selective adsorbent exhibit an advantageous alternative to the standard hydrodesulfurization processes.

EXAMINATION OF THE PRIOR ART

Patent U.S. Pat. No. 5,454,933 describes a process for desulfurization of gas oil that consists in linking a standard hydrotreatment for eliminating the so-called “easy sulfur” sulfur-containing compounds (that can be translated by sulfur-containing compounds that are easy to eliminate) with a process for adsorbing difficult sulfur-containing compounds (called “hard sulfur” in the cited patent) on active carbon with a specific surface area of between 800 and 1200 m²/g and that exhibits a certain porous structure.

These sulfur-containing compounds that are difficult to eliminate most often correspond to beta-substituted dibenzothiophene-type aromatic compounds.

The adsorption process that is described in this patent is limited to temperatures that are less than or equal to 175° C., whereas the process according to this invention operates at temperatures of greater than 200° C. In contrast, the regeneration of the active carbon is carried out in the patent that is cited by means of a solvent, whereas in the process according to this invention, the regeneration of the adsorbent solid is done by a controlled combustion.

Patent Application US 2004140244A1 describes a deep desulfurization of gas oil by using an adsorbent solid with a zinc oxide base that employs a promoter in reduced state, whereas the process according to this invention uses a promoter in oxide form, which makes it possible to carry out the regeneration of the adsorbent solid without having to reduce it.

SUMMARY DESCRIPTION OF THE FIGURES

FIG. 1 shows the diagram of the process for deep desulfurization treatment, the object of this invention.

FIG. 2 shows the diagram of the regeneration of the adsorbent solid used in the process that is the object of the invention.

FIG. 3 shows a non-limiting example of integration of the deep desulfurization process according to the invention with a standard process for hydrodesulfurization of gas oil.

SUMMARY DESCRIPTION OF THE INVENTION

The invention can be defined as a process for deep desulfurization of a gas oil-type hydrocarbon feedstock, with a distillation interval of between 150° C. and 450° C.

In a first step, the process consists in bringing into contact the feedstock to be treated with an adsorbent solid that contains at least zinc oxide, and another metal oxide, called a promoter, deposited on an inorganic substrate under high temperature conditions, typically higher than 200° C., preferably higher than 300° C., and even more preferably higher than 350° C.

In a second step, when the adsorbent solid is saturated by sulfur-containing compounds, the process moves on to the regeneration phase of the solid, the latter essentially consisting of a controlled combustion of compounds adsorbed on the surface of the solid. The regenerated solid can then be brought back into contact with the feedstock that is to be treated.

The industrial use of the process will therefore require at least two units that will be called adsorbers below, working alternately in adsorption phase and in regeneration phase.

The feedstock that is to be treated is a hydrocarbon fraction with a distillation interval of between 150° C. and 450° C., and whose sulfur content should be less than 500 ppmS, preferably less than 200 ppmS, and even more preferably less than 50 ppmS, so as to keep the longest cycle time possible.

The contact between the feedstock to be treated and the adsorbent solid should be made in the presence of hydrogen. The amount of hydrogen to be supplied is set by the ratio of the volumetric flow rate of hydrogen, taken under the input conditions, to the liquid volumetric flow rate of the hydrocarbon feedstock that is to be treated. This ratio should be between 5 and 400 and preferably between 50 and 300.

The contact between the adsorbent solid and the feedstock that is to be treated under the required conditions brings about the passage of a portion of said solid from the oxide phase to the sulfide phase.

Once the adsorbent solid is saturated with sulfur-containing compounds or once the sulfur content of the effluent exceeds the specifications, which specifically means that the adsorbent solid is saturated with sulfur-containing compounds, the adsorbent solid of the contactor that is considered is stripped with nitrogen to eliminate the gases and the hydrocarbons that are partially converted and that are found in the pore volume of said contactor, before entering into the regeneration phase per se.

The process therefore requires at least two adsorbers, working alternately in adsorption/reaction and in regeneration, according to the technique called “swing” reactors that is well known to one skilled in the art.

The stripping phase is accompanied by a rise in temperature that makes it possible both to improve the stripping itself and to reach the necessary temperature for bringing back the adsorption solid of the sulfide phase to the oxide phase by a controlled combustion.

Once this temperature is reached, dilute air is sent into the adsorber so as to carry out the controlled combustion of the adsorbed compounds on the adsorbent solid and thus to bring back the adsorbent solid into oxide phase.

Once the regeneration is terminated, the adsorbent solid is stripped again to eliminate the possible unburned compounds that remain at the end of the burning phase and to drop the temperature again to the level required by the adsorption phase.

The sulfur content of the gas oil that is obtained will generally be less than 10 ppmS, preferably less than 5 ppmS, and even more preferably less than 1 ppmS, with a weight yield relative to the input feedstock that is generally greater than 95% and preferably greater than 97%.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be better understood by following the process diagrams shown in FIGS. 1 and 2.

A diagram example that contains 6 adsorbers numbered from A-1 to A-6 is presented. To make possible a continuous cycle operation, the adsorbers A-1, A-2 and A-3 are in adsorption phase while the adsorbers A-4, A-5 and A-6 are in regeneration phase. Once the cycle time is reached, each adsorber takes the place of the one whose number is immediately above it, except for adsorber A-6 that takes the place of adsorber A-1.

The cycle time is defined as being the perforation time divided by the number of adsorption columns. The perforation time is defined as being the time during which a gas oil is produced to the specifications required for an adsorbent solid volume that is equal to the sum of the volumes of the adsorbers that work in the adsorption phase.

FIG. 1 exhibits the adsorption portion of the deep desulfurization process. The feedstock that is to be treated (1) is mixed with the hydrogen (2) that can be obtained from a hydrotreatment or a hydrogen production unit for forming the flow (3).

The gas/liquid mixture (3) is reheated to the working temperature that is desired in a heat exchanger E-1.

The working temperature is selected based on the nature of the sulfur-containing compounds that are to be treated. In the case of sulfur-containing compounds such as thiophene, the temperature should be greater than 200° C., whereas in the case of the benzothiophenic compounds, the temperature will preferably be greater than 300° C., and in the case of refractory sulfur-containing compounds of the benzothiophenic type, the temperature will preferably be greater than 350° C.

This temperature should, however, be less than the degradation temperature of the feedstock that is used, which is typically 450° C. for a gas oil.

The work pressure will be selected in an interval of between 2 and 20 bar, and preferably between 5 and 15 bar. The amount of solid to be used depends on the VVH (hourly volumetric flow rate) that is defined as the ratio between the liquid volumetric flow rate of the feedstock relative to the volume of adsorbent solid that is used.

The VVH will be between 0.1 h-1 and 10 h-1, and preferably between 0.5 h-1 and 5 h-1.

The effluent flow (4) from exchanger E1 can be divided into, for example, three flows (4 a), (4 b) and (4 c) that will each feed an adsorber A-1, A-2 and A-3.

The effluents (5 a), (5 b) and (5 c) that are obtained from adsorbers A-1, A-2, and A-3 are mixed with one another, and the resulting flow (5) is introduced into a flash tank that works by lowering temperature to separate the hydrogen and to recover the gas oil that is desulfurized to the required specifications.

FIG. 2 exhibits the stages for regeneration of the solid that is used for the deep desulfurization process according to the invention. The adsorber A-4 undergoes a stripping/reheating that makes it possible to evacuate the gas oil feedstock that is present in the pores of the solid. To do this, a cover gas such as the input nitrogen (6) is added to the nitrogen (17 a) that is recycled at the input of the process.

The nitrogen flow (7) that is thus formed is compressed with the aid of the compressor E-2.

The flow (8) that is obtained is separated into two flows. The flow (9) is reheated in the exchanger E-3, and the nitrogen flow (10) that is obtained is used to carry out the stripping and the reheating of the adsorber A-4.

The effluent (11) that is obtained at the outlet of the adsorber A-4 is cooled in the exchanger E-4, and the cooled flow (12) is separated in the separator E-5.

At the bottom of separator E-5, a gas oil flow (13) is recovered, and at the top of separator E-5, a flow (14) is recovered in a first step that consists of a mixture (15) of nitrogen and hydrogen, then in a second step consisting of nitrogen only (16).

The various compounds that remain adsorbed in the adsorbent solid that is contained in the adsorber A-5 are burned by combustion with air that is most often dilute. To carry out this combustion, an addition of dilute air (23) is mixed with the recycled air (33).

The flow (24) that is obtained is compressed in the compressor E-9, and the flow (25) that is obtained is reheated in the piece of equipment E-10.

The flow (26) of dilute and reheated air is used to regenerate the adsorber A-5 by controlled combustion.

The combustion effluents (27) that are obtained and that essentially contain SOx and CO2 are cooled in the exchanger E-11, and the flow (28) is washed in a smoke treatment unit E-12.

The thus treated gases (29) are cooled in the exchanger E-14, and the flow (30) is separated in the separator tank E-14.

The water (31) that is used in the treatment unit E-14 as well as the air that is contaminated by residual smoke (32) are obtained. Residual smoke (34) is removed from the air (32) so as to be able to recycle the clean dilute air (33) at the inlet of compressor E-9.

The adsorber A-6 is cooled under nitrogen. The nitrogen flow (18) that is obtained from flow (8) is cooled in the exchanger E-6.

The flow (19) is used to cool the adsorber A-6. The effluents (20) are cooled in the exchanger E-7. The flow (21) that is obtained is separated in the separator tank E-8. The nitrogen (17) that is recycled at the inlet of the compressor E-2 is recovered at the top of tank E-8, and smoke (22) is recovered at the bottom of tank E-8.

The sulfur content of the feedstock that is to be treated should be less than 500 ppmS, preferably less than 350 ppmS, and even more preferably less than 50 ppmS.

Consequently, a preferred manner of using the deep desulfurization process that is described above is to position it downstream from a standard catalytic hydrodesulfurization process (HDS), which will be able to treat feedstocks with a sulfur content that is optionally greater than 500 ppm and to bring the latter back into the range from which this process can be operated.

It is recalled that a hydrodesulfurization unit (denoted HDS in abbreviated form) transforms the bulk of the sulfur-containing compounds that are contained in a distillate into H2S under temperature conditions of close to 450° C., and pressures of between 20 and 60 bar. However, a portion of the sulfur-containing compounds is called “refractory” with HDS, because their transformation into H2S requires clearly higher pressure and temperature conditions.

These refractory molecules are part of the family of the alkylated dibenzothiophenic compounds.

The content of post-HDS refractory compounds depends on several parameters including the level of pressure and temperature of the hydrodesulfurization unit, the amounts of catalysts involved, and the VVH used.

The content of refractory compounds in the effluent of an HDS unit is in general greater than 15 ppmS, and generally less than 500 ppmS, and even 350 ppmS or 50 ppmS.

In the case where the sulfur content of the hydrotreated gas oil is less than 500 ppmS, it is possible to allow in an additional amount of sulfur in the form of H2S up to the limit of 500 ppmS.

The sulfur content in the form of H2S depends on the operating conditions of the HDS as well as the individual operations (separation, cooling, washing, . . . ) located downstream from the HDS reactor.

FIG. 3 exhibits a typical diagram example of a gas oil HDS.

This diagram is intended to illustrate the different possibilities of positioning of this invention relative to an existing HDS process.

The feedstock that is to be treated (1) is preheated by an effluent-feedstock exchanger E-1.

The preheated feedstock (2) is mixed with the hydrogen (7 a) to form a reheated gas-liquid mixture (8) in the furnace E-3. The feedstock (9) is sent into the HDS reactor E-2 at the desired temperature.

In FIG. 3, a staged HDS reactor is used, in which a portion of the hydrogen (7 b) is added. The output effluents (10) are cooled with the aid of the feedstock-effluent exchanger E-1.

The partially cooled effluents (11) are mixed with the input hydrogen (3) to form a mixture (4) that feeds a gas-liquid separator E-4. The gases (5) are sent to a treatment with amines E-5 that makes it possible to withdraw a portion of the H2S that is present and to deacidify it. The treated hydrogen (6) is compressed in the compressor E-6 to be brought to the desired working pressure in the HDS (7).

The liquid (12) that is collected from the separator E-4 is sent into a separator E-7 to separate the diesel fuel (13) and the water (14).

The gas oil that is obtained (15) is cooled in the exchanger E-8 and the flow (16) that is obtained is sent into a stripper E-9 in which it is mixed with the high-pressure water vapor (17). This makes it possible to recover light hydrocarbons (18) at the top and a clean gas oil without H2S (19) at the bottom.

The flow (19) is cooled in the exchanger E-10, and the flow (20) that is obtained is dried in the piece of equipment E-11. A partially desulfurized gas oil (21) is then obtained at the bottom of the drying apparatus, and water (22) is then obtained at the top of the drying apparatus.

The process for deep desulfurization, object of this invention, can be placed on any of the flows (4), (10), (11), (12), (15), (16), (19), (20) and (21) that are found downstream from the HDS reactor E-2, provided that the total sulfur content of said flow is less than 500 ppmS.

In a preferred manner, the deep desulfurization process will be placed at a location where the content of formed H2S will be the lowest possible, and the temperature will be the highest possible, and even more preferably greater than 200° C.

It is possible, for example, to place the deep desulfurization unit according to the invention at the level of flow (19) that is indicated in FIG. 3.

The adsorbent solid that is used in the deep desulfurization unit generally comprises at least the zinc oxide in the presence of a desulfurization promoter in oxide form, whereby the zinc oxide and the promoter are deposited on a porous substrate or mixed with said porous substrate.

The particles of adsorbent solid will generally be spherical in shape, with a diameter of greater than 500 microns. In some cases, they can have the shape of extrudates, the length of the extrudate in this case being greater than 500 microns.

The particles can have more elaborate shapes than the one called a “trilobe,” which exhibits three protuberances in the form of petals. In a general manner for these various non-spherical shapes, an equivalent diameter, in terms of SAUTER, is defined as being the diameter of the sphere that has the same surface to volume ratio as the one of the particle being considered.

Let us recall that the implementation of the process according to the invention can be done in a fixed bed or in a moving bed. The Sauter diameter of the particles will generally be between 500 microns and 6 millimeters, and preferably between 1 and 5 millimeters.

The amount of elementary zinc is to be greater than 1% by weight, preferably greater than 5% by weight, and even more preferably greater than 10% by weight of the total solid.

The promoter is a metal in oxide form that is selected from among the elements of the following list: titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper or molybdenum.

The promoter will preferably be selected from among the iron or the copper.

The amount of promoter varies between 1 and 40% and preferably between 10% and 20% by weight of the total solid.

The inorganic porous substrate is used to provide the specific surface area whose value is to be greater than 50 m2/g, and preferably greater than 100 m2/g.

The porosity of the substrate is to be essentially of mesoporous type (diameters of pores between 20 and 500 angstroms) and of macroporous type (greater than 500 angstroms) with values of the pore volume of between 0.1 ml/g and 0.6 ml/g for each type of porosity, and preferably between 0.2 and 0.4 ml/g for each type of porosity. It can be selected from among the following list: alumina, silica or a mixture of the two.

The zinc can also be incorporated into the substrate by forming, for example, zinc aluminate, zinc silicate or zinc aluminosilicate. In the same manner, the promoter can be incorporated in the substrate to form an aluminate, a silicate or an aluminosilicate of the metal corresponding to the promoter in question.

The working temperature can be between 200° C. and the degradation temperature of the feedstock that is to be treated. Typically, this degradation temperature is close to 450° C. for a gas oil.

The temperature will preferably be greater than 300° C. and even more preferably greater than 350° C.

The working pressure will be between the atmospheric pressure and 60 bar. Work will preferably be performed on a mixture of feedstock/hydrogen that is for the most part evaporated, and preferably completely evaporated.

EXAMPLE ACCORDING TO THE INVENTION

20 g of an adsorbent solid based on zinc oxide (25% by weight), copper oxide (15% by weight) and alumina oxide (60% by weight) that is loaded into a small laboratory reactor called “catatest” by one skilled in the art is used.

The adsorbent solid particles are spherical and have a diameter of 1.5 mm.

A liquid feedstock comprising decane-type model molecules that represents the gas oil matrix, and dibenzothiophene at a concentration of 200 ppmS that is representative of the refractory sulfur-containing compounds remaining after a hydrodesulfurization (HDS) are considered.

The operating conditions are as follows: Liquid feedstock flow rate: 0.2 l/H Hydrogen flow rate: 40 Nl/h Temperature: 350° C. Total pressure: 10 bar VVH: 4h-1

The test that is carried out makes it possible to produce 60 volumes of desulfurized gas oil with 8 ppm of S per 1 volume of solid used in the reactor. At the end of the test, the solid was partly sulfurized, and the amount of collected sulfur is on the order of 10 mg of S/gram of solid. The gas oil yield is greater than 99%.

After the collection phase, the adsorbent solid is introduced into a flushed bed that is heated to 450° C. under nitrogen. The solid is flushed by a gas mixture that contains 2% by volume of oxygen in the nitrogen for 3 hours, which makes it possible to return the solid to the oxide phase.

The solid is flushed with the nitrogen to cool it and to return to ambient temperature.

The solid can then be used again for the adsorption.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 05/01.968, filed Feb. 25, 2005 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. Process for deep desulfurization of a hydrocarbon fraction with a distillation interval of between 150° C. and 450° C. and of any chemical composition, containing less than 500 ppm of sulfur, for producing a desulfurized fraction that contains less than 10 pm of sulfur, preferably less than 5 ppm and even more preferably less than 1 ppm, whereby said process is characterized in that an adsorbent solid that contains at least one zinc oxide and a promoter in the form of another metal oxide are used, whereby this metal is different from zinc, zinc oxide and the promoter are deposited on a porous substrate based on silica or alumina or a mixture of silica and alumina, said adsorbent solid is brought into contact with the feedstock that is to be treated at a temperature of greater than 200° C. and in the presence of hydrogen until the saturation of said solid in sulfur-containing compounds is completed, then said adsorbent solid is regenerated by a controlled combustion with dilute air so as to regain the oxide form of said adsorbent solid that is then reintroduced in adsorption phase.
 2. Process for deep desulfurization of a hydrocarbon fraction according to claim 1, wherein the zinc that is contained in the adsorbent solid represents more than 1% by weight of the adsorbent solid, preferably more than 5% by weight of the adsorbent solid, and even more preferably more than 10% by weight of the adsorbent solid.
 3. Process for deep desulfurization of a hydrocarbon fraction according to claim 1, wherein the promoter that is contained in the adsorbent solid is selected from among the following metal oxides: titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, molybdenum, and preferably is selected from among the iron or copper oxides.
 4. Process for deep desulfurization according to claim 1, wherein the promoter that is contained in the adsorbent solid represents from 1 to 40% by weight of the adsorbent solid and preferably from 10 to 20% by weight of the adsorbent solid.
 5. Process for deep desulfurization according to claim 1, wherein the porous substrate has a specific surface area that is greater than 50 m²/g and preferably greater than 100 m²/g.
 6. Process for deep desulfurization according to claim 1, wherein the temperature in the adsorption phase is greater than 200° C., preferably greater than 300° C., and even more preferably greater than 350° C.
 7. Process for deep desulfurization according to claim 1, wherein the operating pressure in the adsorption phase is between 1 and 60 bar.
 8. Process for deep desulfurization according to claim 1, wherein the volumetric ratio of the introduced hydrogen, taken under the input conditions of the adsorber, to the feedstock to be treated is between 5 and 400, and preferably between 50 and
 300. 9. Process for deep desulfurization according to claim 1, wherein the porous substrate exhibits both a mesoporosity with a pore volume of between 0.1 and 0.6 ml/g, and a macroporosity with a pore volume of between 0.1 and 0.6 ml/g.
 10. Process for deep desulfurization according to claim 9, wherein the mesoporosity pore volume is preferably between 0.2 and 0.4 ml/g, and the macroporosity pore volume is preferably between 0.2 and 0.4 ml/g.
 11. Process for deep desulfurization according to claim 1, wherein the zinc oxide is incorporated in the substrate in the form of zinc aluminate or zinc silicate or zinc silicoaluminate.
 12. Process for deep desulfurization according to claim 1, wherein the feedstock that is to be treated consists of a hydrocarbon flow that is obtained from a standard hydrodesulfurization unit, provided that the sulfur content of said flow is less than 500 ppm.
 13. Process for deep desulfurization according to claim 1, wherein the feedstock that is to be treated consists of a hydrocarbon flow that is obtained from a hydrodesulfurization unit, provided that the sulfur content of said flow is less than 500 ppm, and the temperature of said flow is greater than 200° C.
 14. Process for deep desulfurization by adsorption on an adsorbent solid of oxide form, a hydrocarbon fraction of the gas oil type, containing less than 500 ppm by weight of sulfur-containing compounds, wherein the yield by weight of the desulfurization is greater than 95% and preferably greater than 97%.
 15. Process for deep desulfurization of a hydrocarbon fraction according to claim 1, wherein the particles of adsorbent solid have a SAUTER diameter of between 500 microns and 6 mm and preferably of between 1 mm and 5 mm.
 16. Process for deep desulfurization of a hydrocarbon fraction according to claim 14, wherein the particles of adsorbent solid have a SAUTER diameter of between 500 microns and 6 mm and preferably of between 1 mm and 5 mm. 